Drive force control method for four-wheel drive vehicle

ABSTRACT

In a drive force control method for a four-wheel drive vehicle using an estimated drive torque for the control, a delay element is added to a value for the estimated drive torque at the trailing edge thereof. With this control, the behavior of the four-wheel drive vehicle is stabilized when a depression force applied to an accelerator pedal is removed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive force control method for afour-wheel drive vehicle.

2. Description of the Related Art

In the case of turning a corner having a small turning radius in afour-wheel drive mode of a four-wheel drive vehicle in a low to mediumvehicle speed range, a difference in rotational speed due to adifference in turning radius is generated between the front and rearwheels of the vehicle, causing a tight corner braking phenomenon. As theprior art for eliminating such a tight corner braking phenomenon, frontand rear wheels driving devices are disclosed in Japanese PatentPublication Nos. 7-61779 and 7-64219.

The front and rear wheels driving devices disclosed in thesepublications have such a structure that a speed increasing device isprovided between main drive wheels and auxiliary drive wheels to therebyadjust an average rotational speed of the auxiliary drive wheels to anaverage rotational speed of the main drive wheels. This speed increasingdevice includes a lockup clutch and a speed increasing clutch, which areselectively switched between ON and OFF states to thereby obtain alockup condition where the average rotational speed of the main drivewheels and the average rotational speed of the auxiliary drive wheelsare substantially equal to each other or a speed increase conditionwhere the average rotational speed of the auxiliary drive wheels isgreater than the average rotational speed of the main drive wheels.

Particularly in the front and rear wheels driving device disclosed inJapanese Patent Publication No. 7-61779, a torque distribution ratiobetween right and left rear wheels are controlled according to a vehiclespeed and a steering angle so that the rear wheel torque is larger thanthe front wheel torque and the turning outer wheel torque is larger thanthe turning inner wheel torque. In this front and rear wheels drivingdevice, the auxiliary drive wheels are increased in rotational speed bythe speed increasing device in turning a corner having a small turningradius in the four-wheel drive mode, thereby preventing the tight cornerbraking phenomenon.

When the vehicle is accelerated during turning, the vertical loads onthe inner wheels and the front wheels are reduced by the influence oflateral and longitudinal accelerations acting on the vehicle body.Further, since the front wheels are steered for turning, a lateral forceacting on the front wheels is greater than that acting on the rearwheels. The greater the vertical load, the greater the drive force thatcan be generated by each tire. Therefore, the load on the tire of eachfront wheel is greater than the load on the tire of each rear wheelduring turning at acceleration, and the load on the tire of each innerwheel is greater than the load on the tire of each outer wheel duringturning at acceleration. The load on each tire depends on the degree ofturning (the magnitude of lateral G) and the magnitude of acceleration.

It is effective to make the load on each tire uniform in improving theacceleration performance during turning. However, no mention as tomaking uniform the vertical load on each tire, or the load on each tireis made in the above publications, and minute drive force control is notdisclosed in these publications. For example, a conventional drive forcecontrol method for the above front and rear wheels driving device has aproblem such that the actual behavior of the vehicle body cannot besufficiently grasped according to a road condition.

The present applicant has proposed a drive force control method for afour-wheel drive vehicle which can overcome the above noted problems aspatent application No. 2004-105023. In this previously filed invention,the control method include estimating a drive torque and carrying out adrive force control method for a four-wheel drive vehicle by using anestimated drive torque.

When the operational condition of the vehicle during turning is shiftedfrom a driving condition where an accelerator pedal is depressed to anengine brake condition where a depression force applied to theaccelerator pedal is removed, a drive force acting on the vehicle isreduced and a deceleration G is generated in the vehicle body. Owing tothis deceleration G and a turning lateral acceleration generated duringturning, the vertical load on each tire changes. As a result, there is apossibility of tuck-in (such that the vehicle shifts inward from adesired turning circle) or track-out (such that the vehicle shiftsoutward from a desired turning circle). In the drive force controlmethod for the four-wheel drive vehicle described in the above priorart, the drive force distribution between the right and left wheels isperformed. Accordingly, when the depression force applied to theaccelerator pedal is removed, the drive force distribution ratio betweenthe right and left wheels is changed to cause a possibility of adverseeffects on the behavior of the vehicle.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a driveforce control method for a four-wheel drive vehicle which can suppress achange in behavior of the vehicle occurring in removing the depressionforce applied to the accelerator pedal.

In accordance with an aspect of the present invention, there is provideda drive force control method for a four-wheel drive vehicle using anestimated drive torque for the control, the drive force control methodcomprising the step of adding a delay element to a value for theestimated drive torque at the trailing edge thereof.

According to the aspect as defined above, a change in behavior of thevehicle in rapidly removing a depression force from the acceleratorpedal can be reduced to thereby improve the drivability of the vehicle.In particular, a track-out phenomenon (such that the vehicle shiftsoutward from a desired turning circle) occurring in removing adepression force from the accelerator pedal can be suppressed.

In accordance with another aspect of the present invention, there isprovided a drive force control method for a four-wheel drive vehicleincluding a torque distributing mechanism capable of changing a driveforce distribution ratio between front and rear wheels and a drive forcedistribution ratio between right and left front wheels or between rightand left rear wheels, the drive force control method comprising thesteps of detecting a lateral G to output a lateral G signal; increasingthe drive force distribution ratio of the rear wheels to the frontwheels according to an increase in absolute value of the lateral Gsignal; increasing the drive force distribution ratio of a turning outerwheel as one of the right and left front wheels or one of the right andleft rear wheels to a turning inner wheel as the other according to anincrease in absolute value of the lateral G signal; and adding a delayelement to a value for an estimated drive torque at the trailing edgethereof.

According to the another aspect as defined above, the drive forcedistribution ratio of the rear wheels to the front wheels is increasedwith an increase in absolute value of the lateral G signal, and thedrive force distribution ratio of the turning outer wheel to the turninginner wheel is increased with an increase in absolute value of thelateral G signal. Accordingly, the load on each tire can be madeuniform, and understeer occurring during turning at acceleration can besuppressed to obtain stable acceleration.

Further, a change in behavior of the vehicle in removing a depressionforce from the accelerator pedal can be reduced to thereby improve thedrivability of the vehicle. In particular, a track-out phenomenonoccurring in removing a depression force from the accelerator pedal canbe suppressed.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a power transmitting system for afour-wheel drive vehicle to which the drive force control method of thepresent invention is applicable;

FIG. 2 is a sectional view of a speed increasing device (speed changingdevice) and a rear differential device;

FIG. 3 is a diagram showing the locus of each wheel during turning ofthe vehicle;

FIG. 4A is a diagram showing the transmission of power to the rearwheels during straight running at acceleration;

FIG. 4B is a diagram showing the transmission of power to the rearwheels during turning at acceleration;

FIG. 5 is a block diagram of a control system according to a preferredembodiment of the present invention;

FIG. 6 is a graph showing the relation between lateral G and torquedistribution ratios of the outer wheel and the rear wheels;

FIG. 7 is a flowchart showing the processing of calculating a driveforce distribution ratio between the front and rear wheels and a driveforce distribution ratio between the right and left rear wheels in thepreferred embodiment of the present invention;

FIG. 8 is a graph showing the relation between estimated slip angle andtorque reducing amounts to the outer wheel and the rear wheels;

FIG. 9 is a flowchart showing the detection of a running condition;

FIG. 10 is a flowchart showing the calculation of a target rear wheeltorque;

FIG. 11 is a flowchart showing 4WD control according to the target rearwheel torque;

FIG. 12 is a graph showing the relation between vehicle speed and torquedistribution to the rear wheels;

FIG. 13 is a graph showing the relation between accelerator opening andtorque distribution to the rear wheels;

FIG. 14 is a graph showing the relation between shift position andtorque distribution to the rear wheels;

FIG. 15 is a graph showing the relation between rear differential oiltemperature and torque distribution to the rear wheels;

FIG. 16 is a flowchart showing the processing of calculating a targetrear outer wheel torque;

FIG. 17 is a graph showing the relation between vehicle speed and torquedistribution to the rear outer wheel;

FIG. 18 is a graph showing the relation between shift position andtorque distribution to the rear outer wheel;

FIG. 19 is a graph showing the relation between rear differential oiltemperature and torque distribution to the rear outer wheel;

FIG. 20 is a flowchart showing the processing of controlling the changefrom a lockup condition to a speed increase condition;

FIG. 21 is a flowchart showing the processing of controlling the changefrom a speed increase condition to a lockup condition;

FIG. 22 is a flowchart showing the processing of stabilizing thebehavior of the vehicle in an unstable condition of the vehicle;

FIG. 23 is a graph showing the relation between shift position andpermission/inhibition of the speed increase;

FIG. 24 is a flowchart showing the control in an engine brake condition;

FIG. 25 is a flowchart showing the control during braking;

FIG. 26 is a flowchart showing the processing of permitting the speedincrease condition after low-speed running;

FIG. 27A is a waveform diagram of estimated drive torque signal with thedelay element added; and

FIG. 27B is a waveform diagram of the trailing edge corrected drivetorque outputted after high selection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a schematic diagram of a powertransmitting system for a four-wheel drive vehicle having a speedincreasing device (speed changing device) 10 based on a front-enginefront-drive (FF) vehicle. As shown in FIG. 1, the power transmittingsystem for the four-wheel drive vehicle mainly includes a frontdifferential device 6 to which the power of an engine 2 located at afront portion of the vehicle is transmitted from an output shaft 4 a ofa transmission 4, a speed increasing device (speed changing device) 10to which the power from the front differential device 6 is transmittedthrough a propeller shaft 8, and a rear differential device 12 to whichthe power from the speed increasing device 10 is transmitted.

The front differential device 6 has a structure well known in the art,and the power from the output shaft 4 a of the transmission 4 istransmitted through a plurality of gears 14 and output shafts 16 and 18in a differential case 6 a to left and right front wheel drive shafts 20and 22, thereby driving front wheels. As will be hereinafter described,the rear differential device 12 includes a pair of planetary gear setsand a pair of electromagnetic actuators for controlling the engagementof multiplate clutch mechanisms. The electromagnetic actuators arecontrolled to transmit the power to left and right rear wheel driveshafts 24 and 26, thereby driving rear wheels.

FIG. 2 is a sectional view of the speed increasing device 10 and therear differential device 12 located downstream of the speed increasingdevice 10. The speed increasing device 10 includes an input shaft 30rotatably mounted in a casing 28 and an output shaft (hypoid pinionshaft) 32. The speed increasing device 10 further includes an oil pumpsubassembly 34, a planetary carrier subassembly 38, a lockup clutch 40,and a speed increasing clutch (speed increasing brake) 42.

When the lockup clutch 40 is engaged, the rotation of the input shaft 30is directly transmitted to the output shaft 32 without changes inrotational speed. On the other hand, when the lockup clutch 40 isdisengaged and the speed increasing clutch 42 is engaged, the rotationof the input shaft 30 is transmitted to the output shaft 32 with therotational speed being increased by a predetermined amount. A detailedstructure of the speed increasing device 10 is disclosed in JapanesePatent Application NO. 2002-278836 previously filed by the presentapplicant. The rear differential device 12 located downstream of thespeed increasing device 10 has a hypoid pinion gear 44 formed at therear end of the hypoid pinion shaft 32. The hypoid pinion gear 44 is inmesh with a hypoid ring gear 48, and the power from the hypoid ring gear48 is input to the ring gears of a pair of left and right planetary gearsets 50A and 50B.

The sun gears of the planetary gear sets 50A and 50B are rotatablymounted on a left rear axle 24 and a right rear axle 26, respectively.The planetary carriers of the planetary gear sets 50A and 50B are fixedto the left rear axle 24 and the right rear axle 26, respectively. Ineach of the planetary gear sets 50A and 50B, the planetary gear carriedby the planetary carrier is in mesh with the sun gear and the ring gear.The left and right planetary gear sets 50A and 50B are connected to apair of left and right clutch mechanism (brake mechanisms) 51 providedto variably control the torque of the respective sun gears. Each clutchmechanism 51 includes a wet multiplate clutch (brake) 52 and anelectromagnetic actuator 56 for operating the multiplate clutch 52.

The clutch plates of each wet multiplate clutch 52 are fixed to a casing54, and the clutch discs of each wet multiplate clutch 52 are fixed tothe sun gear of each of the planetary gear sets 50A and 50B. Eachelectromagnetic actuator 56 is composed of a core (yoke) 58, an excitingcoil 60 inserted in the core 58, an armature 62, and a piston 64connected to the armature 62. When a current is passed through theexciting coil 60, the armature 62 is attracted to the core 58 by thecoil 60 to thereby generate a thrust. Accordingly, the piston 64integrally connected to the armature 62 pushes the multiplate clutch 52to thereby generate a clutch torque.

Accordingly, the sun gears of the planetary gear sets 50A and 50B arefixed to the casing 54, and the drive force of the hypoid pinion shaft32 is transmitted through the ring gears, the planet gears, and theplanetary carriers of the planetary gear sets 50A and 50B to the leftand right rear axles 24 and 26. By making variable the currents to bepassed through the left and right exciting coils 60, the output torquesto the left and right rear axles 24 and 26 can be variably controlled.

When the lockup clutch 40 of the speed increasing device 10 is engagedand the left and right exciting coils 60 of the rear differential device12 are off, the left and right clutch mechanisms 51 are disengaged andthe sun gears of the planetary gear sets 50A and 50B therefore idlyrotate about the left and right rear axles 24 and 26. Accordingly, thedrive force (torque) of the hypoid pinion shaft 32 is not transmitted tothe left and right rear axles 24 and 26. In this case, the rear wheelsidly rotate and the drive force from the engine is fully transmitted tothe front wheels, so that this four-wheel drive vehicle runs in atwo-wheel drive mode.

When predetermined amounts of currents are passed through the left andright exciting coils 60 to completely engage the left and rightmultiplate clutches 52 through the pistons 64, the sun gears of theplanetary gear sets 50A and 50B are fixed to the casing 54. Accordingly,the drive force of the input shaft 30 is uniformly divided by theplanetary gear sets 50A and 50B and transmitted to the left and rightrear axles 24 and 26. As a result, this four-wheel drive vehicle runs ina four-wheel drive mode.

In the case of turning a corner having a small turning radius in thefour-wheel drive mode in a medium vehicle speed range, the lockup clutch40 is disengaged and the speed increasing clutch 42 is engaged.Accordingly, the rotational speed of the output shaft 32 is increasedover that of the input shaft 30. The speed increasing rate is about 5%,for example. In such a case that the vehicle is turned in the conditionwhere the rotational speed of the output shaft 32 is increased over thatof the input shaft 30, the rear wheel on the turning outside can berotated faster than the front wheel on the same side, so that the driveforce can be transmitted to the rear wheel on the turning outside, andthe turning performance in the medium vehicle speed range can beimproved.

The loci of the front wheels and the rear wheels during turning of thevehicle will now be described with reference to FIG. 3. Referencenumeral 66 denotes the center of turning, reference numerals 68L and 68Rdenote the left and right front wheels, respectively, and referencenumerals 70L and 70R denote the left and right rear wheels,respectively. It is assumed that the vehicle is turned counterclockwiseabout the center 66. Reference numeral 72 denotes the locus of the frontinner wheel 68L, reference numeral 74 denotes the locus of the frontouter wheel 68R, and reference numeral 76 denotes the average locus ofthe front wheels. Reference numeral 78 denotes the average locus of therear wheels in the engaged condition of the lockup clutch 40, andreference numeral 80 denotes the locus of the rear outer wheel 70R inthe engaged condition of the lockup clutch 40.

In the case of turning at high lateral G as shown in FIG. 3, the slipangle of the rear wheels becomes larger (the cornering force becomeslarger), so that the locus 80 of the rear outer wheel 70R is larger inradius than the average locus of the rear wheels 78 in the engagedcondition of the lockup clutch 40, and the drive force (torque) is nottransmitted to the rear outer wheel 70R. In the four-wheel drive vehicleaccording to the present invention, the speed increasing clutch 42 ofthe speed increasing device 10 is engaged in this case, therebyincreasing the rotational speed of the output shaft 32 by about 5% overthe rotational speed of the input shaft 30. Accordingly, the drive force(torque) can be transmitted to the rear outer wheel 70R. Referencenumeral 82 denotes the locus of the rear outer wheel 70R in the engagedcondition of the speed increasing clutch 42.

Operation modes of the drive force control method according to thepresent invention are shown in Tables 1A and 1B.

TABLE 1A Mode Forward Straight Left turn Left turn Straight (LSD)(lockup) (speed increase) Acceler- Deceler- Acceler- Deceler- Acceler-Deceler- Acceler- Deceler- Element ation ation ation ation ation ationation ation 1 Speed — — — — — — on on increasing clutch 2 Lockup on onon on on on — — clutch 3 Left Medium Small Large Small Small Small SmallSmall clutch 4 Right Medium Small Large Small Large Small Large Smallclutch

TABLE 1B Mode Reverse Straight Straight (LSD) Acceler- Deceler- Acceler-Deceler- Element ation ation ation ation 1 Speed — — — — increasingclutch 2 Lockup on on on on clutch 3 Left Medium Small Large Smallclutch 4 Right Medium Small Large Small clutch

-   -   In the case of right turn, the magnitudes in the element (3) and        the magnitudes in the element (4) are changed.    -   Conditions for turning (lockup):        -   The vehicle speed is less than 30 km/h or greater than 120            km/h.        -   The lateral G is less than 0.075 G.

Conditions for turning (speed increase):

-   -   -   The vehicle speed is 30 to 120 km/h, and the lateral G is            not less than 0.075 G.

Small: 0 to 40 kgfm Medium: 40 to 80 kgfm Large: 80 to 110 kgfm

In Tables 1A and 1B, “Small”, “Medium”, and “Large” indicate themagnitudes of the engaging force of each clutch. “Small” means 0 to 40kgfm, “Medium” means 40 to 80 kgfm, and “Large” means 80 to 110 kgfm. Inthe case that the vehicle speed is less than 30 km/h or greater than 120km/h during turning, the lockup clutch 40 is engaged. Further, also inthe case that the lateral G is less than 0.075 G, the lockup clutch 40is engaged.

In the case that the vehicle speed is 30 to 120 km/h and the lateral Gis not less than 0.075 G during turning, the speed increasing clutch 42is engaged, so that torque transmission to the rear outer wheel isallowed. While the engaging forces of the left and right clutches 52during left turning are shown in Table 1, the magnitudes of the engagingforce of the left clutch 52 may be interchanged with the magnitudes ofthe engaging force of the right clutch 52 in the case of right turning.

FIG. 4A shows the condition where the lockup clutch 40 is engaged atacceleration during straight running. In this condition, the torque istransmitted uniformly to the left and right rear axles 24 and 26. InFIGS. 4A and 4B, torque transmission paths are shown by bold lines. FIG.4B shows the condition where the speed increasing clutch 42 is engagedat acceleration during left turning. In this condition, the engagingforce of the right clutch 52 is controlled to become larger than theengaging force of the left clutch 52, thereby increasing the torquedistribution to the right rear axle 26.

While the operational conditions shown in Table 1 are the generaloutlines of the drive force control method according to the presentinvention, the drive force control method will now be described indetail.

FIG. 5 is a block diagram of a control system according to the presentinvention. This control system has a feed-forward control section 84, afeedback control section 86, and a speed increase control section 88.Engine torque and transmission gear position are input into a block 90in the feed-forward control section 84 to calculate a tire drive force.A vehicle speed detected by a vehicle speed sensor 92 and a steeringangle detected by a steering angle sensor 94 are input into a block 96to calculate an estimated lateral acceleration (estimated lateral G).

A lateral acceleration (lateral G) detected by a lateral accelerationsensor (lateral G sensor) 98 is input into a block 100 to determine alateral acceleration (lateral G). The lateral G output from the block100 is corrected by the estimated lateral G output from the block 96 toobtain a control lateral G signal. This correction is made by averagingthe lateral G signal and the estimated lateral G signal, for example.The control lateral G signal is input into an outer wheel decision block102 to determine which of the right and left rear wheels is an outerwheel. The control lateral G signal is also input into a block 104 tocalculate a torque distribution ratio between the front and rear wheels,and is also input into a block 106 to calculate a torque distributionratio between the right and left wheels.

The vehicle speed detected by the vehicle speed sensor 92, the steeringangle detected by the steering angle sensor 94, the lateral G detectedby the lateral G sensor 98, and a yaw rate detected by a yaw rate sensor110 are input into a vehicle model block 112 in the feedback controlsection 86 to calculate a slip angle of the vehicle. Further, a slipangle threshold is calculated by a block 114 according to the vehiclespeed detected by the vehicle speed sensor 92 and the lateral G detectedby the lateral G sensor 98.

A rear wheel torque reducing amount is obtained by a block 116 accordingto a difference between the slip angle and the slip angle threshold, andan outer wheel torque reducing amount is obtained by a block 118according to this difference. In other words, if the slip angle of thevehicle is greater than a predetermined value, it is determined that thevehicle is in an unstable condition, and the rear wheel distributedtorque and the outer wheel distributed torque are reduced to eliminatethis unstable condition.

An outer wheel signal from the outer wheel decision block 102, a rearwheel distribution ratio signal obtained by correcting an output fromthe block 104 by an output from the block 106, and a rear outer wheeldistribution ratio signal from the block 106 are input into a block 108to obtain a torque distribution ratio between the rear outer wheel andthe rear inner wheel. In a block 91, a delay element is added to thedrive torque calculated by the block 90, i.e., the estimated drivetorque. This addition of the delay element is attained by interposing alow pass filter or by giving a dead time. A greater one of the outputfrom the block 90 and the output from the block 91 is next selected in ablock 93 to obtain a trailing edge corrected drive torque.

A left rear wheel torque command value is generated by a block 120according to the drive torque corrected by the block 93, the left rearwheel torque from the block 108, and the outer wheel torque reducingamount from the block 118, and the left electromagnetic actuator 56 iscontrolled by a left clutch control section 122 according to the leftrear wheel torque command value generated above. Similarly, a right rearwheel torque command value is generated by a block 124 according to thedrive torque corrected by the block 93, the right rear wheel torque fromthe block 108, and the outer wheel torque reducing amount from the block118, and the right electromagnetic actuator 56 is controlled by a rightclutch control section 126 according to the right rear wheel torquecommand value generated above.

A speed increase threshold is calculated by a block 128 in the speedincrease control section 88 according to the vehicle speed detected bythe vehicle speed sensor 92. The estimated lateral G calculated by theblock 96 and the speed increase threshold calculated by the block 128are compared with each other, and it is determined by a block 130 that aspeed increasing condition is to be provided when the estimated lateralG is greater than the speed increase threshold, whereas the lockupcondition is to be provided when the estimated lateral G is not greaterthan the speed increase threshold. A speed increase signal or a lockupsignal from the block 130 is input into a speed increasing devicecontrol section 132 to control the speed increase/lockup of the speedincreasing device 10.

The drive force control method of the present invention will now bedescribed in detail. When the vehicle is accelerated during turning, thevertical loads on the inner wheels and the front wheels are reduced bythe influence of lateral and longitudinal accelerations acting on thevehicle body. Further, since the front wheels are steered for turning, alateral force acting on the front wheels is greater than that acting onthe rear wheels. The greater the vertical load, the greater the driveforce that can be generated by each tire. Therefore, the load on thetire of each front wheel is greater than the load on the tire of eachrear wheel during turning at acceleration, and the load on the tire ofeach inner wheel is greater than the load on the tire of each outerwheel during turning at acceleration.

The load on each tire depends on the degree of turning (the magnitude oflateral G) and the magnitude of acceleration. Owing to this tendency,understeer occurs in the vehicle during turning at acceleration, and therunning locus of the vehicle is deviated to the outside of turn. As aresult, the acceleration performance during turning is limited. It iseffective to make the load on each tire uniform in improving thisacceleration performance. According to the drive force control method ofthe present invention, the torque distribution ratio between the frontand rear wheels is controlled so that the rear wheel torque is increasedwith an increase in lateral acceleration (lateral G), and the torquedistribution ratio between the right and left wheels is controlled sothat the outer wheel torque is increased with an increase in lateral Gas shown in FIG. 6. Thus, the rear wheel torque distribution ratio andthe outer wheel torque distribution ratio are increased with an increasein lateral G. Accordingly, understeer occurring during turning atacceleration can be suppressed to thereby allow stable acceleration.

The torque distribution between the front and rear wheels and the torquedistribution between the right and left rear wheels will now bedescribed in detail with reference to the flowchart shown in FIG. 7. Instep 10 (shown by “S10” in FIG. 7), the lateral G signal from thelateral G sensor 98 is detected. In step 11, the estimated lateral G iscalculated according to the steering angle detected by the steeringangle sensor 94 and the vehicle speed detected by the vehicle speedsensor 92. In step 12, the lateral G signal is corrected by theestimated lateral G signal to calculate the control lateral G. Thiscorrection is performed by averaging the lateral G signal and theestimated lateral G signal, for example.

The use of an output signal from a lateral G sensor as the lateral Gsignal is most general. However, it is known that the output from thelateral G sensor delays from a turning operation by the operator.Further, an actuator for performing the torque distribution generallyhas delay characteristics. Accordingly, if only the output signal fromthe lateral G sensor is used, control delay occurs. To suppress suchcontrol delay, the estimated lateral G is calculated according to thesteering angle and the vehicle speed detected and the output signal fromthe lateral G sensor is corrected by the estimated lateral G signalobtained above according to this preferred embodiment. Since thesteering angle is a turning operation itself by the operator, theestimated lateral G signal can be generated earlier than the outputsignal from the lateral G sensor. As a result, a control command can beearly output to thereby allow quick-response control.

After calculating the control lateral G in step 12, the program proceedsto step 13 to calculate the rear wheel torque and the outer wheel torqueaccording to the control lateral G. In step 14, it is determined whetheror not the vehicle is in an unstable condition. For example, in the casethat the slip angle of the vehicle is greater than a predetermined valueor the change rate of the slip angle is greater than a predeterminedvalue, it is determined that the vehicle is in an unstable condition.These predetermined values may be changed according to the condition ofa road surface. For example, the smaller the coefficient of friction (μ)between a road surface and each tire, the smaller the predeterminedvalues to be set. Accordingly, the unstable condition can be detectedearlier and more accurately.

If the unstable condition of the vehicle is detected, the programproceeds to step 15 to obtain a rear wheel torque reducing amount and anouter wheel torque reducing amount and to correct the rear wheel torqueand the outer wheel torque according to these reducing amounts,respectively. The rear wheel torque reducing amount and the outer wheeltorque reducing amount are increased with an increase in estimated slipangle as shown in FIG. 8. In other words, the unstable condition of thevehicle is corrected in step 15 by making the torque distribution ratiobetween the front and rear wheels greater on the front wheel side andmaking the torque distribution ratio between the right and left wheelssmaller on the outer wheel side.

If the unstable condition of the vehicle is not determined in step 14 orafter the rear wheel torque and the outer wheel torque are corrected inthe unstable condition of the vehicle in step 15, the program proceedsto step 16 to calculate an actuator control value according to the rearwheel torque and the outer wheel torque. This actuator control valueincludes control values for the right and left electromagnetic actuators56 and control values for the lockup clutch 40 and the speed increasingclutch 42 of the speed increasing device 10. In step 17, the right andleft electromagnetic actuators 56 are controlled and whether the speedincreasing device 10 is to become a lockup condition or a speedincreasing condition is controlled according to the above controlvalues. The degree of this speed increase is set so that the rotationalspeed of the output shaft 32 becomes greater by about 5% than therotational speed of the input shaft 30, for example.

A control method for drive force (torque) distribution between the frontand rear wheels of the four-wheel drive vehicle will now be describedwith reference to the flowcharts shown in FIGS. 9 to 11.

Running condition detection processing will now be described withreference to the flowchart shown in FIG. 9. In step 20, a turningcondition is detected. More specifically, the lateral G signal detectedby the lateral G sensor 98 is corrected by the estimated lateral Gcalculated according to the vehicle speed and the steering angle tocalculate the control lateral G.

In step 21, a vehicle speed is detected from the signal from the vehiclespeed sensor 92. In step 22, an accelerator opening or throttle openingis detected. In step 23, a transmission shift position is detected. Instep 24, a transmission reverse range is detected. In step 25, a 4WD oiltemperature, or an oil temperature of the rear differential device 12 isdetected. Target rear wheel torque calculation processing will now bedescribed with reference to the flowchart shown in FIG. 10. In step 30,a rear wheel torque according to the turning condition is calculated. Instep 31, a rear wheel torque correction amount K1 according to thevehicle speed is calculated. In this preferred embodiment, the torquedistribution to the rear wheels is decreased with an increase in thevehicle speed by using the correction amount K1 as shown in FIG. 12.

In step 32, a rear wheel torque correction amount K2 according to theaccelerator opening or throttle opening is calculated. In this preferredembodiment, the torque distribution to the rear wheels is increased withan increase in the accelerator opening or throttle opening by using thecorrection amount K2 as shown in FIG. 13. In step 33, a rear wheeltorque correction amount K3 according to the transmission shift positionis calculated. In this preferred embodiment, the torque distribution tothe rear wheels is decreased by using the correction amount K3 in thecase that the transmission shift position is a low-speed position or ahigh-speed position as shown in FIG. 14.

In step 34, a rear wheel torque correction amount K4 according to thereverse range is calculated. In this preferred embodiment, the torquedistribution to the rear wheels is decreased by using the correctionamount K4 in the case of reverse running. In step 35, a rear wheeltorque correction amount K5 according to the 4WD oil temperature, or theoil temperature of the rear differential device 12 is calculated. Inthis preferred embodiment, the torque distribution to the rear wheels isdecreased with a decrease in the oil temperature of the reardifferential device 12 by using the correction amount K5 as shown inFIG. 15.

In step 36, the rear wheel torque calculated in step 30 is correctedaccording to the correction amounts K1, K2, K3, K4, and K5 to therebycalculate a target rear wheel torque. In step 40 of the flowchartshowing 4WD control in FIG. 11, an actuator control value is calculatedaccording to the target rear wheel torque. In step 41, the actuator iscontrolled according to the actuator control value calculated above.More specifically, the degree of engagement of the right and leftelectromagnetic actuators 56 is controlled according to the controlvalue to thereby control the torque distribution ratio between the frontand rear wheels.

Target rear outer wheel torque calculation processing will now bedescribed with reference to the flowchart shown in FIG. 16. In step 50,a rear outer wheel torque according to the turning condition iscalculated. This turning condition is determined according to thelateral G. In step 51, a rear outer wheel torque correction amount K6according to the vehicle speed is calculated. In this preferredembodiment, the torque distribution to the rear outer wheel is decreasedwith an increase in the vehicle speed by using the correction amount K6as shown in FIG. 17.

In step 52, a rear outer wheel torque correction amount K7 according tothe transmission shift position is calculated. In this preferredembodiment, the torque distribution to the rear outer wheel is decreasedby using the correction amount K7 in the case that the transmissionshift position is a low-speed position or a high-speed position as shownin FIG. 18. In step 53, a rear outer wheel torque correction amount K8according to the reverse range is calculated. In this preferredembodiment, the torque distribution to the rear outer wheel is decreasedby using the correction amount K8 in the case of reverse running.

In step 54, a rear outer wheel torque correction amount K9 according tothe 4WD oil temperature, or the oil temperature of the rear differentialdevice 12 is calculated. In this preferred embodiment, the torquedistribution to the rear outer wheel is decreased with a decrease intemperature of hydraulic fluid for the rear differential device 12 byusing the correction amount K9 as shown in FIG. 19. In step 55, the rearouter wheel torque calculated in step 50 is corrected according to thecorrection amounts K6, K7, K8, and K9 to thereby calculate a target rearouter wheel torque.

Further, as in step 40 of the flowchart shown in FIG. 11, an actuatorcontrol value is next calculated according to the target rear outerwheel torque calculated above, and as in step 41 in FIG. 11, the degreeof engagement of the right and left electromagnetic actuators 56 arecontrolled according to the control value calculated above. Thelockup/speed increase control for the speed increasing device 10 willnow be described. The object of the lockup/speed increase control forthe speed increasing device 10 is to operate the speed increasing device10 so that the outer wheel can be driven during turning.

Accordingly, the lateral G signal is used to quickly and accuratelydetermine the turning condition. In a straight running condition of thevehicle, the lateral G is zero. Accordingly, by using a small value as alateral G threshold, the speed increasing device 10 can be controlled toa speed increase condition immediately after the vehicle starts turning.For example, when the lateral G signal for the vehicle exceeds thelateral G threshold according to the vehicle speed, the lockup conditionof the speed increasing device 10 is changed to the speed increasecondition. As a result, the speed increasing can be performed beforelargely driving the outer wheel to thereby ensure a condition where theouter wheel can be driven. Accordingly, a larger drive force can beapplied to the outer wheel as compared with the inner wheel, therebyimproving the turning performance.

Further, by using the estimated lateral G signal calculated according tothe steering angle and the vehicle speed as the lateral G signal, thelateral G signal can be obtained more quickly during the process oftransition from the straight running condition to the turning condition.The steering angle is an input itself from the operator, and a delay ofmotion of the vehicle is added to the actual generation of lateral G. Incompensating for the drawbacks of a lateral G sensor, it is alsoeffective to partially correct the output signal from the lateral Gsensor by using the estimated lateral G signal or to use the average ofthe lateral G signal and the estimated lateral G signal.

A speed increase command is generated after the decision of turning. Ifthe speed increasing device 10 is operated immediately according to thespeed increase command, the controller is influenced by the noiseincluded in the signal, and a speed increase stop command is generatedevery time the turning direction changes as in slalom running, causingan increase in frequency of operation of the speed increasing device 10.In order to minimize the noise, shock, etc. due to the operation of thespeed increasing device and reduce the frequency of operation of thespeed increasing device with a reduced size and weight, the speedincreasing device 10 is controlled so that the command to the speedincreasing device 10 is not immediately executed, but the command iscontinued for about one second, for example, prior to performing theactual operation of the device 10.

This control will now be described with reference to the flowchartsshown in FIGS. 20 and 21. FIG. 20 shows the flowchart of change controlfrom the lockup condition to the speed increase condition. In step 60,it is determined whether or not a speed increase command is ON. If thespeed increase command is ON, the program proceeds to step 61 to starttime measurement by a timer. In step 62, it is determined whether or notthe measured time T is greater than a predetermined value T0.

If T>T0 in step 62, the program proceeds to step 64 to determine thespeed increasing operation. Then, the lockup clutch 40 of the speedincreasing device 10 is disengaged and the speed increasing clutch 42 isengaged. If the measured time T is less than or equal to thepredetermined value T0 in step 62, the program proceeds to step 63 todetermine whether or not the speed increase command is OFF. If the speedincrease command is not OFF, the determination of step 62 is executedagain, whereas if the speed increase command is OFF, the determinationof step 60 is executed again.

Change control from the speed increase condition to the lockup conditionwill now be described with reference to the flowchart shown in FIG. 21.In step 70, it is determined whether or not a lockup command is ON. Ifthe lockup command is ON, the program proceeds to step 71 to start timemeasurement by a timer. In step 72, it is determined whether or not themeasured time T is greater than a predetermined value T0. If T>T0 instep 72, the program proceeds to step 74 to determine the lockupoperation. Then, the speed increasing clutch 42 of the speed increasingdevice 10 is disengaged, and the lockup clutch 40 is engaged.

If the measured time T is less than or equal to the predetermined valueT0 in step 72, the program proceeds to step 73 to determine whether ornot the lockup command is OFF. If the lockup command is not OFF, thedetermination of step 72 is executed again, whereas if the lockupcommand is OFF, the determination of step 70 is executed again. Theobject of this speed increase control is to improve the maneuverabilityof the vehicle by driving the outer wheel more than the inner wheel.When the vehicle becomes an unstable condition, there is a case that anyparticular improvement in the maneuverability is not desired under anycircumstances such as counter steer running.

For example, when the slip angle of the vehicle body becomes greaterthan a predetermined value or when counter steer such that the steeringangle and the lateral G are different in sign is detected, the speedincrease control is inhibited. Accordingly, outer wheel driving that mayinvite a further degradation in behavior can be avoided to thereby allowthe stabilization of behavior. Such behavior stabilization control willnow be described with reference to the flowchart shown in FIG. 22. Instep 80, it is determined whether or not counter steer is detected. Ifthe counter steer is detected, the program proceeds to step 82 togenerate a lockup command, thereby engaging the lockup clutch 40 of thespeed increasing device 10.

If the counter steer is not detected in step 80, the program proceeds tostep 81 to determine whether or not the slip angle β of the vehicle bodyis greater than a slip angle threshold β0. If the slip angle β isgreater than the threshold β0, it is determined that the behavior of thevehicle is unstable, and the program proceeds to step 82 to generate thelockup command, thereby engaging the lockup clutch 40 of the speedincreasing device 10 to stabilize the behavior. In such circumstancesthat an improvement in driving stability is not desired or that a largeeffect cannot be obtained by the outer wheel driving as control, thespeed increase control is inhibited to thereby allow a reduction intorque to be input into the speed increasing device 10 and a reductionin frequency of operation of the device 10. Accordingly, this iseffective in reducing the weight of the device 10 and in improving thedurability of the device 10.

For example, when the shift position is a first-speed position or afifth-speed position, the speed increase control is inhibited as shownin FIG. 23. That is, when the shift position is a first-speed position,a very large torque is generated. However, since the vehicle speed atthe first-speed position is low, the effect by the outer wheel drivingcannot be so obtained. Conversely, when the shift position is afifth-speed position, the vehicle speed is too high and there is adanger that the vehicle is excessively turned. Therefore, the speedincrease control is inhibited also in this case. In addition, when theshift position is in a reverse position, an improvement in drivingstability cannot be expected and the speed increase control is thereforeinhibited.

Further, in an engine brake condition or during braking where the driveforce cannot be transmitted to the outer wheel, the speed increasecontrol is also inhibited to thereby allow a reduction in torque to beinput into the speed increasing device 10 and a reduction in frequencyof operation of the device 10. Accordingly, the weight of the device 10can be reduced and the durability of the device 10 can be improved.Further, by controlling the speed increasing device 10 into the lockupcondition in the engine brake condition or during braking, a brakingforce can be applied to the outer wheel, and this is effective also insuppressing oversteer occurring in braking during turning.

Such control in the engine brake condition or during braking will now bedescribed with reference to the flowcharts shown in FIGS. 24 and 25.FIG. 24 shows the flowchart of control in the engine brake condition. Instep 90, it is determined whether or not the drive torque is negative,that is, whether or not the vehicle is in the engine brake condition. Ifthe vehicle is in the engine brake condition, the program proceeds tostep 91 to generate a lockup command, thereby disengaging the speedincreasing clutch 42 of the speed increasing device 10 and engaging thelockup clutch 40.

FIG. 25 shows the flowchart of control during braking. In step 100, itis determined whether or not the vehicle is being braked by theoperator. If the vehicle is being braked by the operator, the programproceeds to step 101 to generate a lockup command, thereby disengagingthe speed increasing clutch 42 of the speed increasing device 10 andengaging the lockup clutch 40. In the case that the operation of thespeed increasing device 10 is relied on the oil pressure of a pumpdriven by an axle, there is a possibility that an oil pressure requiredfor the speed increasing cannot be obtained at certain low vehiclespeeds. If the control is relied on only the lateral G threshold, aspeed increase command is undesirably generated in the stage where asufficient oil pressure is not obtained, causing a possibility ofadverse effects on the speed increasing clutch 42.

Further, when the vehicle speed becomes a value at which a sufficientoil pressure can be obtained, the lockup condition is shifted to thespeed increase condition. Accordingly, even during turning at thisvehicle speed or higher, the lockup condition is changed to the speedincrease condition. To avoid possible instability of the behavior of thevehicle because of the above control, the change to the speed increasecondition is inhibited until the vehicle runs straight at a givenvehicle speed (V1) or more during low-speed running at a given vehiclespeed (V0) or less. Accordingly, the speed increase control at thevehicle speed V0 or less can be avoided. Further, a rapid change to thespeed increase condition during turning can also be prevented.

This control will now be described with reference to the flowchart shownin FIG. 26. In step 110, it is determined whether or not the vehiclespeed V is less than the given vehicle speed V0. If the vehicle speed Vis less than the given vehicle speed V0, the program proceeds to step111 to inhibit the change to the speed increase condition. Thereafter,the vehicle continues to run. In step 112, it is determined whether ornot the vehicle speed V is greater than V1 which is greater than V0 andthe lateral G is less than G0. If the answer in step 112 is YES, theprogram proceeds to step 113 to permit the change to the speed increasecondition. The value G0 in step 112 is set to about 0.1 G. Further, thedetermination in step 112 is to determine whether or not the vehicle isrunning straight at a vehicle speed greater than V1.

The delay element addition in the block 91 and the high select operationin the block 93 shown in FIG. 5 will now be described with reference toFIGS. 27A and 27B. In FIG. 27A, the estimated drive torque calculated inthe block 90 is shown by a solid line, and the delay element added drivetorque obtained in the block 91 is shown by a broken line. In otherwords, the solid line in FIG. 27A illustrates the calculated estimateddrive torque, and the broken line shows the delay, and the right sideportion of the delay (broken line) is added to the trailing edge TE ofthe calculated estimated drive torque to form the trailing edgecorrected drive torque. The delay time is suitably set to hundreds ofmilliseconds, for example. In the block 93, a higher one of the signaloutput from the block 90 and the signal output from the block 91 isselected. Accordingly, the output from the block 93 becomes as shown inFIG. 27B, in which the trailing edge corrected drive torque isillustrated.

In this manner, a predetermined delay element is added to a value forthe estimated drive torque at the trailing edge thereof, therebyallowing the stabilization of the vehicle behavior in removing adepression force from the accelerator pedal. In other words, a change inbehavior of the vehicle in rapidly removing a depression force from theaccelerator pedal can be reduced to thereby improve the drivability ofthe vehicle. In particular, a track-out phenomenon (such that thevehicle shifts outward from a desired turning circle) occurring inremoving a depression force from the accelerator pedal can besuppressed.

While the present invention is applied to a four-wheel drive vehiclebased on a FF vehicle in the above preferred embodiment, the controlmethod of the present invention is also applicable to a vehicle suchthat the power from a driving power source such as an engine is directlytransmitted to the rear wheels, that the transmission of the power tothe right and left rear wheels can be controlled by a clutch or thelike, and that the power can also be transmitted to the front wheels bya clutch or the like. Further, the vehicle may be of such a type thatthe rear wheels are normally increased in rotational speed.

1. A drive force control method for a four-wheel drive vehicle forcontrolling a torque distribution between left and right wheels of oneof a front and a rear set of left and right wheels, said drive forcecontrol method comprising the step of: calculating an estimated drivetorque of the vehicle; generating a trailing edge corrected drive torqueby adding a delay to said calculated estimated drive torque at atrailing edge of the calculated estimated drive torque; and controllingsaid torque distribution between said left and right wheels, based uponsaid trailing edge corrected drive torque.
 2. A drive force controlmethod for a four-wheel drive vehicle including a torque distributingmechanism capable of changing a drive force distribution ratio betweenfront and rear wheels and a drive force distribution ratio between rightand left front wheels or between right and left rear wheels, said driveforce control method comprising the steps of: detecting a lateral G tooutput a lateral G signal; calculating an estimated drive torque of thevehicle; increasing the drive force distribution ratio of said rearwheels to said front wheels according to an increase in absolute valueof said lateral G signal; generating a trailing edge corrected drivetorque by adding a delay to said calculated estimated drive torque at atrailing edge of the calculated estimated drive torque; and increasingthe drive force distribution ratio of a turning outer wheel as one ofsaid right and left front wheels or one of said right and left rearwheels to a turning inner wheel as the other according to an increase inabsolute value of said lateral G signal and the trailing edge correcteddrive torque.